Preparation of Multifunctional Metal Oxynitride 2D Crystals and Oriented Transparent Free-Standing Oxynitride Films

Metal oxynitride two-dimensional (2D) crystals prepared by the delamination of layered compounds have attracted attention as photocatalysts, electrode catalysts, and electrolyte materials. However, the preparation of such materials is a significant challenge because of the difficulty related to producing chemically stable layered oxynitrides. In this study, oxynitride 2D crystals having triple perovskite units (A site: Ca, B site: Ta) were successfully prepared via the perfect delamination of the chemically stable Ruddlesden–Popper phase layered oxynitrides, Na2Ca2Ta3O9N. The layered compound was found to be stable during proton exchange in 6 M HNO3 and a subsequent delamination process, and the resulting oxynitride 2D crystals (herein [Ca2Ta3O9.5N0.5]−1.5) were observed to have a yellow coloration along with a band gap of 2.58 eV. The [Ca2Ta3O9.5N0.5]−1.5 2D crystals modified with RhCrOx and CoOx cocatalysts restacked with alkali ions exhibited photocatalytic activity for H2 and O2 evolution from pure water under UV light and visible light illumination. A transparent, free-standing oxynitride 2D-crystal film having a thickness of around 1 μm was prepared from the 2D-crystal suspension. This film was perfectly oriented in the [001] direction, which resulted in flexibility, mechanical strength, and airtightness due to face-to-face lamination of the 2D crystals. The proton conductivity of the oxynitride 2D-crystal film was around 10–3–10–4 S/cm at 25–50 °C (RH: 90–100%). The cell exhibited a power density of 9 mW/cm2 at 25 mA/cm2. This type of free-standing, flexible oxynitride film is expected to provide a new possibility and to enable the use of oxynitride materials in various applications.